I'm using a 700-ish mhz Dell tower with a front-mounted momentary contact power switch. There's a ribbon cable, 20-conductor or so, leading from the motherboard to a small circuit board mounted in the front case of the computer.
This little circuit board has a few LEDs (power, hard drive access) and two switches (power, reset).
I used a multimeter to figure out which pins on the motherboard are connected to the power switch. Then unplugged the ribbon cable and plugged in a two-pin connector to those two pins on the motherboard.
I drilled a small hole in the case and extended these wires to a Happ pushbutton on the top of my case.
The PC is plugged into a BITS smartstrip, so when the top-mounted button is pressed, everything comes on.